Comprehensive Overview of Computer Networks: Layers, Protocols, and Key Concepts

1. Computer Network Overview

Networks surround us at all times, from telephone and television networks to computer networks, and even internal biological networks like the nervous system. The most typical example is the computer network, which combines computer and communication technologies.

1.1 Classification of Computer Networks

By scope: WAN, MAN, LAN; By users: public networks, private networks.

1.2 Layered Structure of Computer Networks

Comparison of the TCP/IP four‑layer model with the OSI seven‑layer architecture.

1.3 Basic Principles of Layer Design

Layers are independent of each other.

Each layer must have sufficient flexibility.

Layers are completely decoupled.

1.4 Performance Indicators

Rate (bps), delay (transmission, propagation, queuing, processing), round‑trip time (RTT).

2. Physical Layer

The physical layer connects devices and transmits bit streams, providing a reliable physical medium for upper‑layer protocols.

Key devices: Repeater (amplifier), Hub (multi‑port repeater).

Channel concepts: simplex, half‑duplex, full‑duplex.

3. Data Link Layer

3.1 Overview

Provides reliable transmission over the physical layer, handling framing, flow control, error detection, and retransmission.

3.2 Error Detection

Parity check, CRC.

3.3 Maximum Transmission Unit (MTU)

Limits the size of frames; path MTU is determined by the smallest MTU on the route.

3.4 Ethernet Protocol Details

MAC address (48‑bit unique identifier). Ethernet frame structure includes type, source/destination MAC, payload (46‑1500 bytes), and CRC.

4. Network Layer

Implements end‑to‑end data forwarding, addressing, and routing. Core protocol: IP (IPv4/IPv6). Supporting protocols: ARP, RARP, ICMP, IGMP.

4.1 IP Protocol Details

IP provides an unreliable, connectionless service; fields include version, header length, total length, TTL, and protocol identifier (e.g., TCP, UDP).

4.2 IP Forwarding Process

Illustrated flow of packet forwarding through routers.

4.3 Subnetting

Class A/B/C address structures and subnet mask calculations.

4.4 NAT

Network Address Translation allows multiple private hosts to share a single public IP address.

4.5 ARP & RARP

ARP maps IP addresses to MAC addresses; RARP performs the reverse mapping.

4.6 ICMP

Used for error reporting and diagnostics (e.g., ping, traceroute).

4.7 Routing Overview

Routing algorithms: link‑state (LS) using Dijkstra, distance‑vector (DV) using Bellman‑Ford. Protocols: RIP (distance‑vector), OSPF (link‑state), BGP (inter‑AS).

5. Transport Layer

Provides end‑to‑end communication, segmenting data and ensuring reliable delivery.

5.1 UDP

Connectionless, unreliable, minimal header (8 bytes). Header fields: source port, destination port, length, checksum.

5.2 TCP

Connection‑oriented, reliable, flow‑controlled, and congestion‑controlled protocol.

Key functions: segmentation/reassembly, flow control (sliding window), congestion control (slow start, congestion avoidance, fast retransmit, fast recovery), and error detection.

TCP three‑way handshake:

First: SYN from client → server

Second: SYN‑ACK from server → client

Third: ACK from client → server

TCP four‑way termination:

FIN from client → server

ACK from server → client

FIN from server → client

ACK from client → server

6. Application Layer

Provides network services to applications. Main protocols: FTP, Telnet, DNS, SMTP, POP3, HTTP, HTTPS.

6.1 DNS

Translates domain names to IP addresses; resolution order: browser cache, hosts file, router cache, DNS servers.

6.2 DHCP

Automatically assigns IP addresses to hosts on a LAN.

6.3 HTTP & HTTPS

HTTP operates over TCP (port 80); HTTPS adds TLS/SSL encryption (port 443).